The Faintest Stars

[GeeKay]

Does anyone know what the faintest recorded magnitude of a given star happens to be - that is, in terms of absolute magnitude (as opposed to apparent magnitude)? I understand that Jupiter's absolute magnitude is +26. This being so, have there been stars observed that are intrinsically fainter than this planet?

 

On a semi-unrelated point, do dwarf stars (of whatever hue) exhibit fierce gravitational tides near their surface? I ask this because I vaguely recall reading an old SF short story that had a spaceship performing a low altitude flyby past a red dwarf, during which the astronauts experienced no adverse effects from any such notional tidal pull. They wouldn't have found it such a breeze had they tried it on with a neutron star or a stellar-size black hole, of course. But then these two classes of stellar objects weren't around when this story was written. Even so, dwarf stars are no slouches when it comes to size and compression and as such, I just wonder if the author - whose name escapes me at present - got his Isaacs right.

[Ophiolite]

I don't see how a star, producing energy from fusion, no matter how little, could fail to be brighter than a planet whose excess energy output was the result of gravitational contraction. Am I missing something?

[Airbrush]

75% of stars in our galaxy are Red Dwarfs and are invisible to the naked eye. Red Dwarfs do not have greater density than any ordinary star. Red Dwarfs are just small stars, not to be confused with White Dwarfs which are very high density.

[GeeKay]

Airbrush: I am genuinely surprised to learn that white dwarf stars are much more dense than the red dwarf variety. I'd assumed the colour difference was due (for the most part) to age and temperature.

 

Ophiolite: I admit to being a bit hazy about how a star is actually defined. However, I'm referring to those compact (Earth-sized?) dwarf stars, which as far as I know, radiate light etc mainly via gravitational compression, rather than through fusion. It's these fainter stars my original question about absolute magnitudes was trying to address. My mention of Jupiter in this context turned out to be something of a red herring - my apologies.

[Airbrush]

"Airbrush: I am genuinely surprised to learn that white dwarf stars are much more dense than the red dwarf variety. I'd assumed the colour difference was due (for the most part) to age and temperature."

 

There are also brown dwarfs which are ordinary density "failed stars". They are bigger than Jupiter, but not big enough for fusion to begin in their core. Some day, in the distant future there will be "black dwarfs" which are white dwarfs (remnants of dead stars the size of our sun) that burned out entirely. I did a little wikisearch and found also "blue dwarfs".

 

"...According to computer simulations, the minimum mass a red dwarf must have in order to become a red giant is 0.25 solar masses; lesser massive objects, as they age, increase their surface temperatures and luminosities becoming blue dwarfs and from that finally become white dwarfs^[8]"

 

^{http://en.wikipedia.org/wiki/Red_dwarf}

Edited September 25, 2013 by Airbrush

[GeeKay]

Many thanks, Airbrush, and again for the link. I hadn't realised dwarf stars came in so many different masses and colours. I shall resist positing the existence of Snow White stars, deeming such comments to be beneath the dignity of this forum.

 

Incidentally, I did manage to resolve the ambiguity concerning a perceived gravitational tidal pull exerted by (high mass) dwarf stars - Sirius B, for example. Fortunately, even for such a star sporting a surface gravity of around a third of a million gees, any notional tidal pull was calculated to be negligible - a fraction of a G, in fact. . . fortunately in this case, because the SF writer concerned (see above) turned out to be none other than the late great Arthur C Clarke.

 

Apart from brown dwarfs then, I guess non-pulsar neutron stars must be in terms of absolute magnitude among the faintest stars currently in the cosmos. I will stick my neck out here and suggest anything upwards of +30 mag?